This invention relates to a new and improved downcomer and flapper valve particularly useful for transferring overflow particles from an upper fluidized bed of particles to a lower fluidized bed of particles.
The downcomer and flapper valve of the present invention may be incorporated into apparatus for practicing the NOXSO Process disclosed in U.S. Pat. No. 4,798,711 entitled PROCESS FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES AND HYDROGEN SULFIDE FROM GAS STREAMS, patented Jan. 17, 1989, Lewis G. Neal, et al., inventors, and assigned to the same assignee as the present invention; this patent is hereby incorporated herein by reference and will be referred to hereinafter as the "'711 patent." Generally, the NOXSO Process disclosed in the '711 patent uses SO.sub.x and NO.sub.x from gas streams particularly coal combustion flue gas. alumina sorbent beads or particles for the removal by adsorption of As illustrated in FIG. 3 of the '711 patent, the sorbent particles contact the flue gas in the adsorber 14 having a fluidizing bed 15 and the SO.sub.x and NO.sub.x in the flue gas are adsorbed onto the surface of the sorbent particles and thereby removed from the flue gas. Sorbent particles loaded with the SO.sub.x and NO.sub.x are transported to a multi stage fluid bed heater 18 where the loaded sorbent particles are contacted by hot air to remove or strip the NO.sub.x from the particles. Thereafter, the sorbent particles now having only the SO.sub.x adsorbed thereto are transported to a moving bed regenerator 32 where the sorbent particles are contacted with a regenerant gas stream 36 which reacts with the adsorbed SO.sub.x to produce elemental sulfur contained in the off-gas stream 38. Subsequently, the sorbent particles are transported to a multi-stage fluid bed sorbent cooler 52 where the sorbent particles are contacted with cooled air to reduce the temperature of the previously heated particles. The sorbent particles have then been regenerated and the SO.sub.x and NO.sub.x adsorption removal process is repeated.
FIG. 1 is a diagrammatical illustration of a prior art multi-stage fluid bed heater which may be utilized at the multi-stage fluid bed heater 18 shown in FIG. 3 of the '711 patent. The multi-stage fluid bed heater shown in FIG. 1 provides three beds of fluidized sorbent particles, 21, 22 and 23 residing above perforated metal fluidizing grids 25, 26 and 27 and disposed vertically in the metal vessel 30; the perforated metal fluidizing grids 25, 26 and 27 are suitably secured to the metal vessel 30 such as by belting or welding. The sorbent particles enter the top portion of the vessel 30 as indicated by the arrow 32. Hot air, as indicated by the arrow 33, enters the bottom of the vessel 30 and flows upwardly through each of the three fluidized beds of sorbent particles 21, 22 and 23 to remove or strip the NO.sub.x from the sorbent particles which NO.sub.x exits the vessel 30 in a gas stream indicated by the arrow 34. Tubular metal overflow pipes 36 and 38 are mounted to and extend upwardly from the fluidizing grids 25 and 26 and tubular metal downcomers 40 and 42 are suitably mounted to and extend vertically downwardly from the fluidizing grids 25 and 26. For example, the overflow pipe 36 and downcomer 40 are coextensive tubular members and may either be formed separately and suitably joined or may be a single tubular member. In operation, sorbent particles in the fluidized bed 21 in excess of the height of the overflow pipe 36 flow into and downwardly through the overflow pipe and further downwardly through the downcomer 40 and into the fluidized bed 22. Sorbent particles in excess of the height of the overflow pipe 38 flow into and downwardly through the overflow pipe and downwardly through the downcomer 42 into the fluidized bed 23. Sorbent particles are removed from the bottom fluidized bed 23 utilizing a flow controlled L-valve 44, of the type known to the art, and are removed at a rate sufficient to maintain a constant level in the bottom fluidized bed 23; sorbent particles exit the vessel 30 through the L-valve 44 as indicated by the arrow 45. It will be further noted from FIG. 1 that the downcomers 40 and 42 extend downwardly into the fluidized beds 22 and 23 and that the bottoms of the downcomers 40 and 42 open into the bodies of the fluidized beds of sorbent particles 22 and 23.
Each downcomer 40 and 42 is provided with a flapper valve 50 which may be of the type shown in FIGS. 2A through 2C. It will be generally understood that the flapper valves 50 are closed when no sorbent particles are flowing downwardly from an upper bed to a lower bed and that the flapper valves 50 prevent air flow back through the downcomers when the downcomers are not full of sorbent particles. This condition occurs on start-up when the sorbent particles reside above the fluidizing grids and have not yet been fluidized. If the flapper valves 50 were not present at the bottoms of the downcomers sufficient air would flow through the downcomer such that the sorbent particles residing above the fluidizing grids would not fluidize and the beds would not fill with sorbent particles. The flapper valve 50 includes a rectangular tubular member 51 including a tapered lower portion 52 provided with a vertical opening 54 better seen in vertical cross-section in FIG. 2C. A flapper valve 56 is mounted pivotally at 57 to the downcomer 50 at a position to cause the flapper valve 56 to engage the downcomer and normally close the opening 54. It has been found that the prior art flapper valve 56 has at least three major limitations. The flapper valve 56 tends to hang open allowing gas flow up through the downcomer to which it is mounted thereby preventing sorbent particle flow. Secondly, the vertically oriented opening 54 requires the sorbent particles to flow horizontally out of the downcomer. This horizontal flow, combined with the relatively small cross-sectional area of the opening or outlet 54, limits the maximum flow rate of sorbent particles through the downcomer. Finally, it has been found that flow stops through the downcomer 50 if the level of the fluidized bed into which the bottom portion of the downcomer 50 extends rises to the top of the flapper valve 56 where the flapper valve is mounted pivotally to the downcomer 50; in this condition, the flapper valve substantially ceased to open and flow of sorbent particles out of the opening 54 was prevented.
Accordingly, there exists a need in the art for a new and improved downcomer and flapper valve which provides improved performance and overcomes the above-noted limitations associated with the prior art flapper valves of the type shown in FIGS. 2A-2C.